The present invention relates to a plasma-arc work process using a plasma torch supplied with a gas mixture of an oxidizing gas and nitrogen, the amount of one gas in the mixture with respect to the other being defined according to one or more parameters chosen from the thickness and/or the grade of the constituent material of the workpiece, the desired work rate and the intensity of the electric current.
A plasma-arc torch that can be used in a cutting, welding, marking or spraying operation, or in any other heat-treatment operation, on a metallic or nonmetallic material usually comprises an electrode made of copper or of a copper alloy carrying a cylindrical insert generally made of hafnium, tungsten or zirconium, on which the electric arc serving to ionize the gas feeding the torch takes root, that is to say the defined flow rate of pressurized gas, called plasma gas, which is delivered between the electrode and the nozzle and which flows out via an orifice of said nozzle toward the workpiece.
The electrode is generally centered above this orifice for ejecting the plasma jet, which is placed axially in the nozzle and which forms a constriction diaphragm.
In the particular case of a plasma cutting operation, the plasma cutting device or system usually comprises a plasma torch, a source of electric current, an electric-arc ignition system and one or more sources of fluids, particularly of plasma gas, possibly of shielding gas or of post-injected fluid, and of fluid for cooling the torch, generally distilled water.
Such torches or units are well known to those skilled in the art, since they have already been described in many documents to which reference may be made for further details, especially EP-A-599709, EP-A-872300, EP-A-801882, EP-A-941018, EP-A-144267, EP-A-410875, EP-A-772957, EP-A-902606, EP-A-810052, EP-A-845929, EP-A-790756, EP-A-196612, WO-A-89/11941, U.S. Pat. No. 4,521,666, U.S. Pat. No. 4,059,743, U.S. Pat. No. 4,163,891 and U.S. Pat. No. 5,591,357.
In a known manner, plasma-arc cutting uses the thermal and kinetic effects of a plasma jet to melt the material to be cut and to expel the molten material from the kerf formed after the torch has moved relative to the workpiece.
Moreover, it is also known that, in plasma cutting, the plasma gas or gas mixture used is not the same depending on the nature of the material to be cut.
Thus, a nonoxidizing gas, for example nitrogen, is generally used for stainless steels or for light alloys, whereas it is preferred to use an oxidizing gas, such as oxygen or air, for cutting structural steels. However, in the case of structural steels, even though compressed air has the advantage of being simpler to use than oxygen, since a simple air compressor is sufficient to supply the torch and there is therefore no need to supply gas bottles, it is found that the air used as cutting gas results in cut faces which are of inferior quality to those obtained using oxygen, that is to say it results in the presence of flash and of striations. Oxygen is therefore usually preferred for this reason.
Moreover, the use of mixtures of nitrogen and oxygen in proportions of about 30% nitrogen, as plasma gas, make it possible to increase the cutting speed by about 20% with respect to pure oxygen.
In addition, this same type of nitrogen/oxygen mixture used as a dual flow enables, in some cases, the roughness of the cut faces to be reduced.
However, this nitrogen/oxygen mixture is usually premixed, that is to say manufactured according to the chosen contents, before being used to fill suitable containers, such as gas bottles, which are then taken to the operating site, that is to say the point where the cutting has to be carried out.
Now, the fact that this type of nitrogen/oxygen mixture must be premixed and contained in gas bottles poses a problem.
Thus, it will be understood that, when a nitrogen/oxygen mixture has to be used with different nitrogen contents in the oxygen in order to comply with several different cutting applications or situations, it is necessary to store as many gas bottles as desired contents, that is to say nitrogen/oxygen mixtures having different nitrogen contents.
This therefore requires the use of a large stock of gas bottles of different compositions suitable for the various situations that might arise and also requires many bottle movements and handling operations.
Under these conditions, it is essential to put into place strict procedures for storing and handling each bottle, as well as precise monitoring of the stocks in order to prevent any shortage in terms of a particular gas mixture, something which is difficult to manage and involves relatively large financial investment and operating costs.
The object of the present invention is therefore to solve these problems of bottle storage and movement while making it easier to use mixtures of nitrogen and oxygen of different contents.
In other words, the aim of the present invention is to provide a plasma-arc work process using nitrogen/oxygen mixtures of variable contents compatible with various arc-work situations, particularly various material-cutting situations, while at the same time maintaining or improving the cutting performance in terms of rate, flash and/or roughness of the cut faces, and, more generally, a plasma-arc work process using nitrogen/oxygen-containing gas mixtures.
The present invention therefore relates to a process for the plasma-arc working of at least one workpiece made of structural steel, in which:
(a) a plasma torch is supplied with an electric current and with a gas mixture containing oxygen and nitrogen,
(b) a plasma jet obtained by the ionization of at least part of said gas mixture containing oxygen and nitrogen by said electric current is delivered by means of said plasma torch, wherein:
the concentration of nitrogen in the gas mixture containing oxygen and nitrogen is greater than 0% by volume and less than 50% by volume,
said gas mixture is obtained by the addition of a defined amount of nitrogen to an oxidizing gas containing oxygen, said addition of nitrogen being carried out immediately before the gas mixture containing oxygen and nitrogen is introduced into said torch, and
the defined amount of nitrogen is defined according to at least one parameter chosen from the thickness of the workpiece, the grade of the constituent structural steel of the workpiece, the desired work rate and the intensity of the electric current.
So-called xe2x80x9cstructuralxe2x80x9d steels are defined by the Comitxc3xa9 Europxc3xa9en de Normalisation [European Standardization Committee] as belonging to the families of nonalloy steels, low-alloy steels or high-alloy steels satisfying the EN 10025, EN 10113-2, EN 10113-3, EN 10137-3 et seq. standards.
According to another aspect, the invention also relates to a process for the plasma-arc working of at least one workpiece made of stainless steel, in which:
(a) a plasma torch is supplied with an electric current and with a gas mixture containing oxygen and nitrogen,
(b) a plasma jet obtained by the ionization of at least part of said gas mixture containing oxygen and nitrogen by said electric current is delivered by means of said plasma torch,
wherein:
the concentration of oxygen in the gas mixture containing oxygen and nitrogen is greater than 0% by volume and less than 25% by volume,
said gas mixture is obtained by the addition of a defined amount of oxygen or of a gas containing oxygen in nitrogen, said addition of oxygen being carried out immediately before the gas mixture containing oxygen and nitrogen is introduced into said torch, and
the amount of oxygen or of oxidizing gas containing oxygen is defined according to at least one parameter chosen from the thickness of the workpiece, the grade of the constituent stainless steel of the workpiece, the desired work rate and the intensity of the electric current.
Steels-called xe2x80x9cstainlessxe2x80x9d steels belong, according to the Comitxc3xa9 Europxc3xa9en de Normalisation [European Standardization Committee] to the family of alloy steels containing at least 11% chromium.
Depending on the case, the process of the invention may include one or more of the following characteristics:
the plasma-arc work process is a plasma cutting or plasma marking, preferably plasma cutting, process;
the grade of the constituent material of the workpiece is chosen from structural steels and stainless steels;
the material making up the workpiece may or may not include a surface coating such as of paint, zinc, aluminum or another protective coating;
the process uses a plasma gas mixture adapted or adaptable to the work to be carried out;
the concentration of nitrogen in the gas mixture containing oxygen and nitrogen is between 10% by volume and 40% by volume, preferably between 20% by volume and 40% by volume, for work on structural steel;
the concentration of oxygen in the gas mixture containing oxygen and nitrogen is between 1% by volume and 22% by volume, preferably between 5% by volume and 20% by volume, for working on stainless steels;
the oxidizing gas is oxygen or compressed air, which is possibly dried and stripped of dust and of oil droplets liable to be generated by compression, and preferably oxygen;
the addition of gas is carried out in at least one gas mixer, preferably said gas mixer includes means for controlling or adjusting the content of the gas to be added;
the amount of gas to be added is defined according to the thickness of the workpiece and to at least one parameter chosen from the grade of the material, the desired work rate and the intensity of the electric current;
the thickness of the workpiece is between 0.4 mm and 20 mm, preferably between 0.5 mm and 12 mm;
the desired work rate is between 0.5 and 10 m/min, preferably between 1 and 5 m/min;
the intensity of the current is between 10 A and 150 A, preferably between 15 and 120 A;
The invention also relates to a unit for the plasma-arc working of at least one workpiece, particularly one capable of implementing a process as mentioned above, which comprises:
at least one plasma torch fastened to a support frame and able to move along at least one axis of movement,
at least one current generator connected to said torch,
at least one source of an oxidizing gas containing oxygen,
at least one nitrogen source,
at least one gas mixer supplied with the oxidizing gas containing oxygen by said at least one source of oxidizing gas and with nitrogen by said at least one nitrogen source, said gas mixer including means for controlling or adjusting the concentration of nitrogen and/or of oxidizing gas containing oxygen, so as to obtain a gas mixture containing oxygen and nitrogen with a nitrogen concentration in said gas mixture greater than 0% by volume and less than 50% by volume, or an oxygen concentration in said gas mixture greater than 0% by volume and less than 25% by volume, the amount of nitrogen or oxygen being defined according to at least one parameter chosen from the thickness of the workpiece, the grade of the constituent material of the workpiece, the desired work rate and the intensity of the electric current,
the gas mixer is placed between said source of oxidizing gas containing oxygen, said nitrogen source, and said torch,
means for controlling the flow rate and/or the pressure of the gas mixture are placed between the outlet of the mixer and the inlet of the torch.
By virtue of the present invention, it is possible, from pure or substantially pure gases and because of the use of a means of adding and controlling either the amount of nitrogen mixed into the oxidizing gas, such as oxygen or compressed air, or the amount of oxidizing gas, such as oxygen or compressed air, mixed into the nitrogen to guarantee, in a plasma cutting machine, a controlled content of each constituent of the nitrogen/oxygen mixture which is adapted to each particular use or operation that has to be carried out.
Precise control of the content of each constituent depending on the type of application makes it possible to increase the plasma cutting performance according to productivity and/or quality criteria.
The nitrogen content in the oxygen or the oxygen content in the nitrogen is controlled automatically by an external control depending on several parameters, such as the thickness of the sheet, the intensity of the current, the grade of material to be cut, the cutting rate, etc.